It has long been known in motor vehicle engineering to optimize combustion processes in a combustion chamber of an internal combustion engine by using fuel injectors. In corresponding internal combustion engines with injection, it is possible, in comparison to internal combustion engines which have a carburetor, to realize a considerably better quantitative accuracy and possibly also a better spatial distribution of the fuel which is introduced into the respective combustion space or intake section.
Fuel injectors usually have a coil which generates a magnetic field when current is applied, it being possible for a magnetic armature to be moved from a closed position to an opening position against the force of a restoring spring by said magnetic field. A valve needle is fitted to the magnetic armature, said valve needle, when the magnetic armature is in the closed position, closing an injection opening in the fuel injector. When the magnetic armature is in the opening position, the injection opening is then opened by the valve needle and pressurized fuel can be output through the injection opening. In order to close the fuel injector, the application of current to the coil is stopped, and as a result the magnetic force on the armature is eliminated. The armature is moved back to its closed position by the remaining force of the restoring spring.
A known eddy current-driven coupling between the mechanism (armature and valve needle) and magnetic circuit (coil) of the fuel injector generates a feedback signal, which is based on the movement of the mechanism, in a known manner. In this case, a speed-dependent eddy current, which likewise causes a reaction on the magnetic circuit, is induced in the armature, which comprises a ferromagnetic material, as a result of the movement of the valve needle and of the armature. Therefore, a voltage, which is superimposed on the drive signal, is induced in the coil as a function of the movement speed of the armature and the valve needle.
It is further known that this effect, in the case of which a change in signal, which is impressed by the movement of the valve needle, is superimposed on the main electrical variable of voltage or current, can be further processed in such a way that the electrical component which is caused by the speed or, more precisely, by the change in the speed of the armature can be separated. In this case, in particular, a characteristic signal in the voltage or current signal is evaluated in respect of the time at which it occurs. Since the change in speed at the time at which the end position is reached is particularly large, it is possible to use this to determine the actual time at which the armature or the valve needle which is attached to the armature reaches the opening position.
In principle, the following methods are known for detecting a characteristic signal profile during the opening process:
(A) Current measurement: this requires the current profile to be actively influenced in order to ensure that the magnetic circuit is not in saturation. However, with this measurement method, a measurement signal first can be detected at full drive, that is to say at the mechanical stop of the valve needle.
(B) Voltage measurement: in this case, it is necessary for the coil to be driven using so-called sample & hold driving with a boost phase. In spite of this, it is generally not possible or at least very difficult, against the background of a relatively large drive voltage, to identify all of the required and typically relatively weak characteristics in the voltage which is applied to the coil and to evaluate them in respect of analysis of the armature movement.
The manufacture of fuel injectors is subject to tolerances. For example, in different fuel injectors, different spring forces and/or different levels of guide play (friction) can occur during opening and closing, said spring forces and levels of guide play in turn leading to different delay times and therefore to different injection quantities.